As brake systems for a vehicle, hydraulic brake systems have been used in which a friction pad is driven by a hydraulic cylinder to be pressed against a brake disk. However, in recent years, with the introduction of brake control such as anti-lock brake system (ABS), electric brake systems are gathering attention in which no hydraulic circuit is used.
In electric brake systems, an electric linear motion actuator is used to convert the rotation of a rotary shaft driven by an electric motor to the linear motion of a linear motion member, and a friction pad driven by this electric linear motion actuator is pressed against a brake disk, so as to generate braking force.
An electric linear motion actuator used in such electric brake systems is disclosed, for example, in the below-identified Patent document 1. The electric linear motion actuator disclosed in Patent document 1 includes a rotary shaft to be driven by an electric motor, a plurality of planetary rollers kept in rolling contact with the cylindrical surface formed on the outer periphery of the rotary shaft, a carrier which retains the plurality of planetary rollers such that the respective planetary rollers are rotatable about their axes while revolving around the rotary shaft and of which the axial movements are restricted, and an outer ring member surrounding the plurality of planetary rollers and supported so as to be slidable in the axial direction. Helical ribs are formed on the inner periphery of the outer ring member, and helical grooves are formed on the outer peripheries of the respective planetary rollers so as to be engaged with the helical ribs.
When the electric motor rotates the rotary shaft, the respective planetary rollers, which are kept in rolling contact with the rotary shaft, revolve around the rotary shaft while rotating about their axes. When the planetary rollers revolve around the rotary shaft, the outer ring member makes a linear motion due to a difference in lead angle between the helical ribs on the inner periphery of the outer ring member and the helical grooves on the outer peripheries of the respective planetary rollers, so that the friction pad is driven by the outer ring member, which makes a linear motion, so as to be pressed against the brake disk.